B11E-01
Incorporation of Root and Surface Litter Inputs Into Soil C Pools: What Do Different Physical Fractionation Approaches Tell Us?
Efforts to isolate soil C pools related to the structure, function and turnover of soil organic matter (SOM) often employ physical fractionation methods. Both density-based and particle-size fractionations have been used to examine the role of aggregates in SOM cycling and stabilization. After a 14C pulse labeling of deciduous forest, reciprocal transplants of enriched vs. near-background litter allowed us to track the source and dynamics of SOM pools isolated by both fractionation approaches. Light fraction (LF) and particulate organic matter (POM) were separated into unprotected and aggregate-protected pools by applying different dispersion energies. The mineral-associated pool was characterized either as the dense fraction (DF) or as particles <53 μm (MOM). We used procedural constraints and the distributions of bulk soil C and 14C to compare pools isolated by each method. The uPOM fraction included both free and macroaggregate-associated particulate C pools, whereas the free LF isolated mostly unprotected, interstitial particulate C. Operationally, the mPOM fraction consisted of microaggregate-protected particulate C, but the occluded LF included both macroaggregate-associated and microaggregate-protected C. Overall, POM accounted for 5% more of bulk soil C than LF, with the difference likely due to inclusion of some microaggregate-protected particulate C in the DF. POM/LF-C dynamics associated with root turnover occurred mostly at the macroaggregate rather than the microaggregate scale during the 4-y study. In contrast, the dynamics of litter C sources occurred at both scales suggesting a different mode of incorporation, e.g., sorption of soluble C to exchange sites or assimilation by microbes associated with the POM/LF. Acid hydrolysis of the MOM fraction revealed dynamic components that rapidly incorporated and cycled C inputs from both root and litter sources. Taken together, the results improve our understanding of short-term soil C dynamics and the C pools isolated by physical fractionation methods.
B11E-02
Rhizosphere Effects Reduce the Temperature Sensitivity of Soil Organic Carbon Decomposition
The potential change in soil CO2 efflux rates in response to warming constitutes a crucial feedback between the global carbon cycle and the climate. The strength of this feedback depends on the realized temperature sensitivity of soil organic carbon (SOC) decomposition. Total soil CO2 efflux originates from two main sources: (1) rhizosphere respiration of plant-derived recently assimilated carbon by roots and associated rhizosphere microbes, and (2) microbial respiration of SOC. The two sources also interact through a process known as the rhizosphere priming effect in which rhizosphere microbial activities may increase respiration of SOC up to fourfold. However, past laboratory studies have omitted the rhizosphere priming effect, while field- warming experiments that measure total soil CO2 efflux cannot discern the temperature sensitivity of rhizosphere respiration from that of SOC decomposition. Little is known about the role of rhizosphere effects in modulating the overall temperature sensitivity of SOC decomposition. We carried out a continuous 13C- labeling experiment and assessed the temperature sensitivity of SOC decomposition with and without rhizosphere effects. Results indicated that the rhizosphere priming effect reduced the temperature sensitivity of SOC decomposition. Rhizosphere respiration was insensitive to 5C soil warming above the ambient temperature regime. The combined effect of rhizosphere priming and rhizosphere respiration resulted in a much reduced temperature sensitivity of the overall CO2 efflux belowground. These results demonstrate that realistic assessment of the temperature sensitivity requires inclusion of rhizosphere effects, because the absence of rhizosphere effects in short-term laboratory soil incubations may cause overestimation of the temperature sensitivity of SOC decomposition.
B11E-03
Soil Warming and Carbon Release: Varying Patterns of Organic Matter Breakdown Across Five Ecosystems
Our understanding of the mechanisms governing soil organic carbon (SOC) retention vs. loss as CO2 in the future suffers from an inability to predict how mineralization of labile vs. recalcitrant SOC will proceed with warming. Incubation and field studies of soil warming have resulted in conflicting conclusions about how multiple SOC pools will respond to rising temperatures. In this study, we explore SOC transformations in a long-term incubation with warming from five North American ecosystems by assessing respired CO2 and solid state 13C nuclear magnetic resonance (NMR) spectra of non-incubated and cool vs. warm incubated soils. We also quantified extra-cellular enzyme activities (EEA) late in the incubation to assess how relatively slow-turnover SOC pools responded to warming. Soils from a cool temperate forest, a warm temperate forest, and a temperate grassland released an average of 95% more CO2 when warmed by the end of the 200 d incubation. Soils from a boreal forest and acidic arctic tundra released slightly more CO2 with warming during the first 20 d of the incubation, but after day 20 and until day 200 CO2 released by warmed soils was equivalent to the control soils. NMR spectra and EEA data suggest varying responses of these ecosystems' SOC pools to warming. Arctic tundra soils did not experience a change in the kinds of C compounds mineralized with warming, yet EEA data indicate greater acquisition of C from phenolic compounds. Boreal forest soil NMR spectra suggest greater net humification with warming, with only slight increases in enzymatic C acquisition. These soils may have experienced an increase in C use efficiency with warming that resulted in acquired C not being "wasted" on respiration. Cool temperate forest soils exhibited no change in the kinds of C accessed by microbes with warming, as revealed via NMR, in spite of these soils' greater C acquisition and respiration with warming. Grassland soils experienced an increase in humification with warming, associated with an increase in CO2 released. Warm temperate forest soils, in contrast, exhibited less humification with warming. These data suggest that changing microbial C use efficiency with warming is an important and relatively unexplored determinant governing the net influence of warming on SOC transformations and eventual CO2 release.
B11E-04 INVITED
A Dual Arrhenius and Michaelis-Menten (DAMM) Kinetics Model of Soil Organic Matter Decomposition
Conceptual and numerical models of decomposition of soil organic carbon (SOC) often assign degrees decomposability, ranging from "labile" to "recalcitrant" substrates, which roughly correspond to ranges of turnover times, from "fast" to "slow." However, this conceptual continuum confounds the effects of complex molecular structures of substrates with other factors that also slow rates of decomposition, such as physical and chemical protection of substrates in soil aggregates and on mineral surfaces. Hence, SOC can be old for a variety of reasons. This confusion has clouded related issues, such as the temperature dependence of decomposition of SOC. Decomposition of old SOC is sometimes reported as unresponsive to temperature, whereas Arrhenius kinetics dictate that decomposition of complex molecular structures should have high activation energies (high temperature dependence). Here we offer a conceptual model of decomposition of SOC substrates using dual Arrhenius and Michaelis-Menten kinetics to show that low substrate supply at microsites of enzymatic activity can obscure the temperature sensitivity dictated by Arrhenius kinetics. Simulated SOC stocks are very sensitive to parameterization of the fractions of substrate pools that are available to diffuse to reaction sites of enzymes (the fraction that is not physically protected). Combining Michaelis-Menten and Arrhenius kinetics with heterogeneous substrate supply provides a conceptual framework that is consistent with both basic principals, as we understand them, and observations of a significant fraction of SOC that cycles slowly.
B11E-05 INVITED
Carbon in Deep Soil: why does it just sit there?
A huge amount of carbon is present in deep soil below the bulk rooting zone; concentrations are low but there is a lot of deep soil. That carbon is also typically very old, with average 14C turnover times of thousands of years. This has raised the question: what is the nature of that material that makes it so apparently recalcitrant? Hypotheses have focused on two core mechanisms: chemical recalcitrance, in which the material at depth is the stuff that microbes couldn"t process and so it accumulates; and physical protection, in which the material is sorbed to mineral surfaces and so is unavailable to microbes. A third possibility is that it is neither-that much of it is potentially degradable but that its lack of processing results from the sparse nature of the deep soil environment, with organic matter and microbes widely spread and thus not readily in contact with each other. Thus a lack of processing may result from the limited transport of molecules to static microbes, exacerbated by the economics of organic matter processing-microbes must invest in making enzymes to break down particular molecules, and if there isn"t enough of a particular substrate, then the 'return-on-investment' may be inadequate to support metabolizing a particular molecule. We explore these ideas through experimental work, in which treated soils from 1 m depth in a California grassland to repeated dry-wet cycles to mobilize C. Under constant 'optimum' moisture, soil respiration rates were close to zero, dry-wet cycles increased total C mineralization by 4-fold, microbial biomass 5-fold, and N mineralization 2- fold. The C released had an average turn-over time of 600-800 years based on 14C dating the CO2. We discuss these and other recent results within the context of the mechanisms that might allow biologically labile to accumulate in deep soils.
B11E-06
New Insights Into Carbon Sequestration of Steppe Soils - Composition and Turnover of Soil Organic Matter Fractions and Aggregation
Grazing is one of the most important factors that may reduce soil organic carbon (SOC) stocks and subsequently aggregate stability in grassland topsoils. Improvements of land use management and grazing reduction are assumed to increase the carbon sequestration of steppe ecosystems which may act as one of the big global carbon sinks. The central aim of this study was to analyse the quantity and quality of SOC fractions and their contribution to aggregate formation, stability and carbon sequestration as affected by increased inputs of organic matter due to grazing exclusion. We applied a combined aggregate size, density and particle size fractionation procedure and aggregate stability measurements to sandy steppe topsoils with different organic matter inputs due to different grazing intensities (continuously grazed = Cg, winter grazing = Wg, ungrazed since 1999 = Ug99, ungrazed since 1979 = Ug79). Higher inputs of organic matter led to higher amounts of OC in coarse aggregate size classes (ASC) and especially in particulate organic matter (POM) fractions. We found no grazing-induced changes of soil organic matter (SOM) quantity in fine ASC and mineral fractions. SOM quality (13C CPMAS-NMR spectroscopy, neutral sugars analyses) was comparable between different grazing intensities, but SOM in ungrazed plots was more decomposed across all fractions. We found generally higher radiocarbon activities in Ug79 compared to Cg. Aggregate stability, analysed as resistance to sonication, was higher in Ug79 compared to Cg. Higher litter inputs in grazing exclosures increased POM quantity, led to faster SOM turnover and resulted in the formation and stabilisation of coarse aggregates. Organo-mineral associations were affected by higher turnover times as radiocarbon activities increased, but OC saturation of this pool did not change. To summarise, additional litter inputs following grazing exclusion were mainly sequestered in the intermediate POM pool while the long-term pool of organo-mineral associations did not change and was close to saturation. We conclude that management changes in steppe ecosystems do not necessarily increase carbon sequestration and their assumed potential to act as carbon sinks has to be questioned.
B11E-07
A new Insight Into Microscale Soil Organic Matter Dynamics - From Single Particles to Aggregates
Both mineral interactions and the spatial inaccessibility due to aggregation are key-factors affecting the stabilization of soil organic matter (SOM). Knowledge about the factors controlling the preservation of SOM and underlying stabilization mechanisms has improved significantly over the last years. Nevertheless, in situ processes remain almost unclear and are still challenging to evaluate. In the presented work, we studied the alteration of spatial distribution of fresh introduced OM over time on single particles and in intact soil aggregates. Single particles of a fine silt and clay mixture (< 6.3 µm) were taken from an Albic Luvisol (Ah horizon, < 2mm) and pre-incubated for 500 days at 20°C. Intact soil aggregates were derived from a Haplic Chernozem (Ap horizon, < 6.3 mm). These two sample materials were selected due to their differences in 14C signatures with high values in single particles (pMC = 104.6) and rather low values in intact aggregates (pMC = 84.6). SOM composition of all samples was characterized by 13C-CPMAS NMR spectroscopy: Single particles were dominated by aliphatic C structures, whereas intact aggregates consisted mainly of aromatic C (char). Both sets of samples were pre-incubated at 20°C. We used a labelled amino acid mixture (approx. 99 atom% 13C and 15N) as readily bioavailable OM input and isotopic tracer. The samples were incubated at 20°C for 6 days and sub-samples were taken consecutively over a 6- days incubation period. Single particles were deposited on silicon wafers, whereas aggregates were resin embedded. Samples were then analyzed by scanning electron microscopy (SEM) and nano-scale secondary ion mass spectrometry (nanoSIMS50). We will demonstrate the spatial distribution of OM on single particles and in intact soil aggregates at the microscale by SEM and nanoSIMS. In addition, with the isotopic sensitivity of nanoSIMS, we are able to follow the fate of 13C and 15N, which is expected to be influenced by diffusion, sorption and microbial activity. From these results, we propose how OM in soil can be stabilized on single soil particles and at complex soil aggregates.
B11E-08 INVITED
Dissolved organic matter production and stabilization across pedogenic thresholds
We examined the composition of dissolved organic matter (DOM) and its interaction with soil minerals across a substrate age gradient in Hawai'i. Across the 4.1 million year gradient, both forest type (Meterosideros polymorpha) and climate (2500 mm MAP) are well constrained, while soils ranging from base cation-rich primary minerals at the youngest site to metastable poorly-crystalline oxyhydroxides at intermediate aged sites to crystalline oxides at the oldest sites. Our objectives were (1) to determine the type and sources of organic matter that accumulates with depth, (2) to assess interactions between soil minerals, metals and DOM, and (3) to assess the possible mechanisms for DOM accumulation. As an initial step in this project we developed a soil column leaching experiment to examine DOM fluxes and chemistry. Sequentially deeper soil samples were collected from 5 sites across a substrate age gradient (300y - 4.1My) of mantle-derived lava. Field moist samples from major organic and mineral soil horizons (O, A, B) were repacked into 10 cm diameter ABS columns individually. A series of column leaching experiments were then performed by percolating rainwater through organic and subsequently mineral soil cores. In addition to measuring fluxes of DOM (both carbon and nitrogen) and CO2, we have measured major cations, anions, UV absorbance, δ13C and bioavailability of DOM. Preliminary results suggest large differences in DOM quantity and possibly quality being produced from the O horizons across the gradient. These differences correspond to known differences in nutrient availability and ecosystem productivity between sites. DOM interactions with the mineral soil varied considerably suggesting that pedogenic state plays a strong role in determining the fate of DOM in mineral soils, including the extent to which interaction, recalcitrance and accessibility influence DOM stabilization in soil.